Many diseases (e.g., cancers, hematopoietic disorders, endocrine disorders, and immune disorders) arise from the abnormal expression or activity of a particular gene or group of genes. Similarly, disease can result through expression of a mutant form of protein, as well as from expression of viral genes that have been integrated into the genome of their host. The therapeutic benefits of being able to selectively silence these abnormal or foreign genes are obvious.
Double-stranded RNA molecules (dsRNA) can block gene expression by virtue of a highly conserved regulatory mechanism known as RNA interference (RNAi). Briefly, the RNA III Dicer enzyme processes dsRNA into small interfering RNA (siRNA) of approximately 22 nucleotides. One strand of the siRNA (the “antisense strand”) then serves as a guide sequence to induce cleavage of messenger RNAs (mRNAs) including a nucleotide sequence which is at least partially complementary to the sequence of the antisense strand by an RNA-induced silencing complex RISC (Hammond, S. M., et al., Nature (2000) 404:293-296). The antisense strand is not cleaved or otherwise degraded in this process, and the RISC including the antisense strand can subsequently effect the cleavage of further mRNAs.